The distance a man is able to comfortably travel (from his bed) without pants; the average radius being just the distance to the shower.
It is theorized that a man's underpants radius is inversely proportional to his confidence; that is, the farther a man travels from his bed in just his underwear, the less he believes in himself.
At first we thought it was funny; at least Marshall didn't left the house without putting pants on. But his underpants radius just kept getting larger. He started getting the paper in his underwear and even going downstairs for a drink in his underwear...!